The invention relates generally to lasers, and particularly to diode pumped solid-state lasers.
Currently, there are two general physical arrangements for diode pumping solid-state lasers. These are "end pumping", in which the diode pump radiation enters the laser medium along its axis, in the same direction as the laser flux propagates; and "side pumping", in which the pump radiation enters the laser medium from the side, at an angle to the direction of laser flux propagation. End pumped lasers are described in U.S. Pat. Nos. 4,653,056 of Bayer & Keirstead, and 4,710,940 of Sipes.
Side pumping is generally thought to be more readily scalable, the accepted method of achieving this including by adding successively more diode pumps along the length of the medium and also encircling the medium with the pump sources. In the latter case, the diode radiation is often focused into the center of a laser rod, to overlap in a circular region and thus increase gain. This method is used to produce a circular gain volume with a gaussian intensity profile to match the desired profile of a TEMoo laser output.
Continuous wave (CW) diode-pumped lasers are typically of much lower gain than pulsed pumped lasers, especially when built with a side-pumped construction. Devices mentioned above, with diodes encircling the laser rod and their outputs focused within the rod, have produced gains of only 8%, compared to gains of 200% for pulsed pumping.
In addition, the cooling of an encircled rod laser device must be achieved with liquid because the heat sources are dispersed three dimensionally; i.e., there is no common contour through which the heat could be extracted conductively by a common metal sheet or surface. Typically, the laser rod is surrounded by a sleeve which flows cooling water, and the diode pump light must pass through this sleeve to reach the laser rod.
Another laser configuration is the slab laser, in which the pumping light is spread uniformly over the side of the laser crystal and the laser beam zig-zags through the laser medium, reflecting multiple times off the pumped faces. This laser obtains better overlap between laser mode and pumped volume, than in the side-pumped laser rod. Inventors have gone to great lengths to develop slab constructions that will improve beam quality and efficiency of sidepumped lasers. For example, diode pumped slab constructions have used end-pumping techniques to match the focused output of a single diode emitter to the cavity mode at each bounce point along the slab, as in U.S. Pat. Nos. 4,908,832 and 4,710,940. These techniques are efficient, but are extremely alignment sensitive. In practice, such systems are limited if power scaling is attempted, because each additional bounce increases the alignment sensitivity of the laser.
Power scaling by increasing the length of the laser crystal to accommodate more diodes, leads to other problems. Longer crystal are more susceptible to bowing and to stresses that introduce birefringence and cause beam aberrations. Linear losses increase with laser crystal length, and become especially significant in lower gain CW-pumped lasers. Increasing the pump power on a single short crystal is therefore more effective than using longer crystals and spreading out the pump power along the crystal length.
Side-pumped lasers generally sacrifice efficiency for beam quality. In rod lasers encircled by diodes whose outputs overlap in the center of the rod, the laser energy near the rod edges, where the diode light first enters the rod, is largely wasted and does not contribute to the laser output. For high efficiency and good beam quality, one must match the mode of the laser, which is determined by the shape of the optical cavity, to the diode pump volume, which is determined by the diode imaging optics and the laser crystal's absorption characteristics. In multimode operation, however, side pumped lasers may be as efficient as end pumped lasers because all of the pumped volume can be filled by multiple cavity modes.
For typical cavities employed in compact lasers, the TEMoo (lowest order, highest beam quality) mode is typically less than 1 mm in diameter. Larger modes can be produced by lengthening the cavity or employing graded reflectivity mirrors, but these changes produce larger losses and reduce efficiency for low gain CW-pumped lasers. To obtain TEMoo mode operation of the laser, the diode pump light must be contained within this less than 1 mm region. In side pumping, the absorption depth of the material is generally larger than 1 mm (typically 3 mm for Nd:YAG), so it is difficult to confine the pump light to the central volume required by the TEMoo mode. Generally, apertures must be employed to confine the laser mode, and such spatial filters typically lower the efficiency for TEMoo operation to about 50% of that achieved in multimode operation.
The prior art patents identified above state that side-pumping, (also called transverse or lateral pumping) does not provide mode matching and is therefore inefficient for producing a low divergence or high quality laser output (i.e., TEMoo mode output). There is clearly a need for a side-pumped laser construction that is simpler, more readily scalable, and capable of providing good beam quality output.